1. Field of the Invention
This invention concerns a method of radiation induced dry etching of a metal substrate. More particularly, the invention concerns the use of a halogen gas which reacts with the metal forming a solid reaction product which is capable of being removed when irradiated with a beam of radiation generated by an excimer laser.
2. The Prior Art
The trend in electronics today is towards systems of ever increasing component density. Increased component density permits designers to achieve greater speed and complexity of system performance while maintaining system size at a minimum. Additionally, increased component density enables manufacturers to lower production costs owing to the economies that can be realized using integrated circuit processing.
The desire for increased component density has given rise to very large scale integrated circuit (VLSI). In such circuits, designers pack large numbers of electrical components onto individual integrated circuit chips. Subsequently, these chips are ganged on a substrate to form larger circuits and functional blocks of a system.
To facilitate the mounting of the high density circuit chips, designers have developed the so-called multilayer ceramic (MLC) substrate. The MLC substrate is well known and has been described in such articles as "A Fabrication Technique for Multilayer Ceramic Modules" by H. D. Kaiser et al, appearing in Solid-State Technology, May, 1972, pp. 35-40.
An example of a semiconductor module including a multilayer ceramic substrate is given in U.S. Pat. No. 4,245,273 issued to Feinberg et al and assigned to the assignee of this application.
MLC manufacturers have found that substrate performance, particularly, the maximum circuit speed the substrate will sustain, can be increased by reducing the length of the thick film metal wiring built into the substrate to interconnect the chips. Designers have proposed to reduce interconnection wiring by replacing at least some of the MLC thick film circuits with multilayer thin film circuits. Particularly, designers have proposed to use thin film circuits at the MLC chip mounting surface. The thin film circuits are formed at the MLC chip mount surface as multiple layers of thin film metal separated by layers of insulation such as a polyimide or other polymeric organic material. The multiple metal layers are interconnected by vertical metallization which extends through holes commonly referred to as vias that are arranged in a predetermined pattern.
Because it is possible to make a line of smaller dimension, using thin film technology as compared with thick film technology, it is possible to fit more circuits in a substrate plane. Where higher circuit density per plane is achieved, fewer planes are required and accordingly the circuit wiring length interconnecting the multiple planes can be reduced. By shortening the plane interconnection metallization less circuit inductance and parasitic capacitance is present permitting the higher frequency performance. This technique for increasing frequency capability has come to be referred to as Thin Film Redistribution (TFR). An illustration of an MLC including a TFR structure is provided in U.S. Pat. No. 4,221,047 issued to Narken et al and assigned to IBM Corporation, the assignee of this invention.
While the size of TFR multilevel metallization structure is smaller than that of thick film, it is not as small as thin film metallization structure used on the chips. Because the TFR current is a combination of the currents supplied by the multiple chips, it is substantially greater than the chip current. The TFR metallization must therefore be of larger physical size than that of the chip to maintain current densities and associated heating at acceptable levels. Additionally, the dielectric separating the TFR metal layers is also thicker and of different composition. As taught in the above mentioned U.S. patents, copper is the metal most widely used for forming the metallization patterns. It is therefore obvious that copper etching is an essential process in both Thin Film Redistribution (TFR) and Metallized Ceramic Polyimide (MCP) technology, and more generally for various packaging applications where there is a need to define wiring patterns in thick copper films.
Unfortunately, because TFR metallization structures are larger than those of an integrated circuit chip and because the materials are somewhat different, the thin film process techniques conventionally used for an integrated circuit chip metallization fabrication such as the lift-off etching technique and dry etching (plasma or reactive ion etching) cannot be easily used in making TFR structures. The lift-off technique is complex and difficult to define in thick films. Dry etching needs complex equipment and process steps involving inorganic masks such as MgO and SiO.sub.2. Furthermore, dry etching is not accurately repeatable and controllable particularly in large batch processing.
U.S. Pat. No. 4,490,211 issued to Chen et al and assigned to IBM Corporation, the assignee of this application, the disclosure of which is herein incorporated by reference, discloses a process for dry etching the copper metallization layers of MCL substrates having TFR multilayer copper metallization layers wherein the metallized copper substrate is mounted in a reaction chamber in which a vacuum of predetermined pressure is established. A halogen gas, such as chlorine is introduced into the chamber. The gas spontaneously reacts with the copper substrate and forms a solid reaction product (CuCl) thereon by partial consumption of the copper surface. The CuCl surface is selectively irradiated with a patterned beam of radiation from a pulsed excimer laser operating at a wavelength suitable for absorption by the CuCl. Whenever the excimer laser strikes, due to heating caused by absorption of the radation, the thin layer of CuCl is vaporized exposing a fresh layer of copper. A new layer of CuCl is formed on the freshly exposed metal, as before, by reacting the metal with additional quantities of the halogen has. This new layer of CuCl, in turn, is removed by irradiating with a pulse of laser radiation. In this manner, the metal is etched.
In areas of the copper metallization which are not irradiated with radiation from the excimer laser, the CuCl reaction product remains intact until removal, at the termination of the laser etch process, by rinsing in a diluted chemical solution such as dilute ammonium hydroxide solution. Due to the selective nature of etching of the copper metal, patterning thereof is possible using the excimer laser radiation.
One drawback to the laser induced chemical etching process disclosed in U.S. Pat. No. 4,490,211 is that the etching process is relatively slow and consumes a considerable amount of laser energy.